Derivatives (Details) In Billions, unless otherwise specified	Mar. 31, 2013 USD ($)	Mar. 31, 2013 JPY (¥)	Dec. 31, 2012 USD ($)	Dec. 31, 2012 JPY (¥)
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Total guarantee value	$ 2.4	¥ 228.5	$ 2.7	¥ 230.0
Collective account values of underlying variable annuities as a percentage of the guarantee value	94.00%	94.00%	87.00%	87.00%

Represents the collective account values of underlying variable annuities as a percentage of the guarantee value of the associated variable annuity reinsurance liability.

No definition available.

Name:	wtm_CollectiveAccountValueAsPercentageOfGuaranteedValue
Namespace Prefix:	wtm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant